The present invention relates to the predistortion of a modulated signal prior to its transmission over a wired or wireless channel, with the purpose of reducing the distortion incurred by the signal from the channel. An example of such a channel is a satellite communication channel, whereby optimal satellite resources usage requires transmitting a signal which uses the satellite amplifier at or close to its saturation point and which exhibits a high bandwidth relative to the bandwidth of the circuitry present in the satellite and ground infrastructure. Under these circumstances, the transmitted signal typically incurs distortion, reducing the reliability of the communication.
The transmitter output in a digital communication system, particularly a satellite communication system, can be seen as a sequence of modulated complex symbols. Each symbol is selected from an allowed set of complex values, represented by an in-phase and quadrature component (I and Q). The set of possible symbols is called a constellation. Several modulation strategies can be envisaged in satellite communications, including quadrature amplitude modulation (QAM), phase shift keying (PSK) and amplitude and phase shift keying (APSK). These modulation strategies employ different type of constellations. For example, in the APSK modulation scheme the constellation points are located on two or more concentric rings.
In FIG. 1 a satellite communication link with its main components is shown as an example of a transmission link. In this example structure, the transmit symbols are upsampled, fed to a transmit filter, modulated onto a carrier wave and upconverted to a frequency suitable for signal transmission to the satellite. The signal is received by the satellite's transponder, which operation is illustrated in the simplified schematic drawing of FIG. 2. The transponder's incoming signal is passed to a bandpass input multiplexer filter (IMUX), amplified by a travelling wave tube amplifier (TWTA), and filtered again by a bandpass output multiplexer filter (OMUX). The transponder output signal travels to a plurality of receivers. One such receiver will downconvert the signal to a frequency convenient for demodulation, demodulate the signal and filter it with a receive filter. Finally, it will sample the signal to extract the received symbols. A transmission link, like the example shown in FIG. 2, can be represented by an equivalent symbol-input/symbol-output model.
In the absence of channel distortion, the output symbols of the transmission link or its equivalent model are equal to the input symbols. On a transmission link of practical use, distortion changes the average location of the received constellation symbols. This kind of distortion is typically caused by channel non-linearities which change the phase and amplitude of the transmitted signal as it passes through the channel. Channel distortion, be it linear or non-linear, also causes adjacent symbols to interfere with each other. This type of interference is known as inter-symbol interference (ISI).
Several prior art predistortion techniques exist that introduce a compensation in the transmitter for the distortion caused by the channel. One of these techniques, disclosed e.g. in WO02/73920, applies successive interference cancellation (SIC). SIC aims at subtracting from the transmitted symbols the interference expected at the receive side. It comprises several identical stages, which successively generate an approximation of the required predistortion in order to compensate for the remaining receiver distortion. This reduces the inter-symbol interference (ISI) and partially corrects the location of the received constellation symbols.
In patent application US2002/164949 a satellite communication apparatus is disclosed comprising a corrector for reducing distortion of a signal in a communication channel. The corrector is formed by a forward model representative of a pre-calculated estimate of non-linearity and group delay of the satellite transmission/reception channel from the input to the up-sampler through to the output of the down-sampler. The forward model may also comprise a model of magnitude response of a satellite input multiplexer filter and output multiplexer filter. Cascaded identical stages of the corrector may be provided so that distortion in the channel is successively reduced towards zero by each successive stage.
However, various problems exist with the prior art solutions. The first problem is that the distortion on the constellation points with a large magnitude is not perfectly corrected. This is because the non-linearity limits the correction applicable close to the saturation point. For APSK constellations, for example, the impacted constellation points are located on the outer ring. A second problem is that the constellation points with large magnitude are biased towards the constellation origin. As a result the average points (or mass points) of each received constellation point do not coincide with the transmitted constellation. A consequence of this problem is that the mass points of the received constellation deviate from the original constellation and therefore the original constellation no longer delivers the performance it was expected to deliver. Another consequence is that a receiver unaware of the channel distortion expects the average points of the received constellation at their original location. As a result, the reliability and performance of the communication system is reduced.
Hence, there is a need for a solution where the above-mentioned problems are avoided or overcome.